This invention relates to low resistance lap joints for superconducting magnet coils, particularly suitable for main magnet coils.
As is well known, a magnet can be made superconductive by placing it in an extremely cold environment, such as by enclosing it in a cryostat or pressure vessel containing liquid helium or other cryogen. The extreme cold reduces the resistance in the magnet coils to negligible levels, such that when a power source is initially connected to the coil (for a period, for example, of ten minutes) to introduce a current flow through the coils, the current will continue to flow through the coils due to the negligible resistance even after power is removed, thereby maintaining a magnetic field. Superconducting magnets find wide application, for example, in the field of magnetic resonance imaging (hereinafter "MRI").
The main magnet coils are wound on a composite drum, which in an MRI may have a circumference in the order of 150 inches. Thus, the magnet coils are relatively large and require a considerable length of wire or conductors. Moreover, in a typical MRI there are six different main magnet coils wound circumferentially around the outer surface of the drum and axially spaced from one another. These coils are connected in series and are normally wound by feeding the conductor off a supply reel while rotating the drum. Because of the superconducting temperatures (in the order of -270.degree. C.) and the conflicting thermal, electrical, magnetic and mechanical considerations and factors which must be considered in an MRI design, the magnet coil conductor may be of relatively expensive material, such as niobium titanium (NbTi) or niobium tin (Nb.sub.3 Sn).
Because of the large size and axial length of the main magnet coils in an MRI superconducting magnet, it becomes necessary to electrically connect the coils in a series circuit. However, it is important that the connection or joint have a reliable uniform low resistance, in the order of less than 1.times.10.sup.-10 ohms. The principle of superconducting current flow is based on the absence of electrical resistance to the current flow at the superconducting temperatures. It is thus extremely important to keep the resistance of electrical connections or joints low. Even a single watt of heating can result in the boiling of 1.4 liters per hour of helium, which is completely unacceptable, since a typical MRI specification limits the helium boil-off to only 0.2 liters per hour. Moreover, MRI magnets are subject to drift if there is any appreciable resistance in the magnet circuit, and MRI drift rate specifications and requirements necessitate very low resistance. In addition, resistance must also be minimized to enable the MRI magnet to operate in the persistent mode and to avoid quenching of the magnetic field after it is initially ramped to field. A superconducting magnet ramping current may be as high as 750 amps. Still further, in placing a superconducting magnet into operation, the temperature of the magnet assembly must be reduced from ambient temperatures to superconducting temperatures, placing significant mechanical and thermal stresses on the magnet coils.
As a result, the superconducting magnet joints must withstand significant thermal, electrical, magnetic and mechanical stresses and factors which must be balanced and compromised in order to obtain an optimum design.